The present invention relates generally to dielectric filters, and, more particularly, to a multi-passband, dielectric filter, such as a duplexer filter, of a design which minimizes the physical dimensions thereof.
Advancements in the field of radio electronics have permitted the introduction and commercialization of an ever-increasing array of radio communication apparatus. Advancements in electronic circuitry design have also permitted increased miniaturization of the electronic circuitry comprising such radio communication apparatus. As a result, an ever-increasing array of radio communication apparatus comprised of ever-smaller, electronic circuitry has permitted the radio communication apparatus to be utilized more conveniently in an increased number of applications.
A radio transceiver, such as a radiotelephone utilized in a cellular, communication system, is one example of radio communication apparatus which has been miniaturized to be utilized conveniently in an increased number of applications. Additional efforts to miniaturize further the electronic circuitry of such radio transceivers, as well as other radio communication apparatus, are being made. Such further miniaturization of the radio transceivers will further increase the convenience of utilization of such apparatus, and will permit such apparatus to be utilized in further increased numbers of applications.
Pursuant to such efforts to miniaturize further the electronic circuitry comprising radio transceivers, as well as other radio communication apparatus, size minimization of the electronic circuitry comprising such is a critical design goal during circuit design.
Dielectric block filters, comprised of a ceramic material, frequently comprise a portion of the circuitry of such radio transceivers. Such dielectric block filters are advantageously utilized for reasons of cost, simplicity of manufacture, ease of installation upon an electrical circuit board, and good filter characteristics at frequencies (typically in the 900 Megahertz and 1.7 Gigahertz range) at which such transceivers usually are operative.
To form a filter of a block of dielectric material, holes are bored, or otherwise formed, to extend through the dielectric block, and sidewalls defining such holes are coated with an electrically-conductive material, such as a silver-containing material. The holes formed thereby form resonators which resonate at frequencies determined by the lengths of the holes.
Typically, substantial portions of the outer surfaces of the dielectric block are similarly coated with the electrically-conductive material. Such portions of the outer surfaces are typically coupled to an electrical ground.
Spaced-apart portions of a top surface of the dielectric block are also typically coated with the electrically-conductive material which is electrically isolated from the electrically-conductive material coated upon other outer surfaces of the dielectric block. Adjacent portions of the electrically-conductive material coated upon the top surface become capacitively coupled theretogether. Additionally, such portions capacitively load respective ones of the resonators.
The resonators, due to electromagnetic intercoupling between adjacent ones of the resonators, the portions of the top surface of the block due to capacitive coupling, and the capacitive loading of the resonators together define a filter having filter characteristics for filtering a signal applied thereto.
The precise filter characteristics of such a filter can be controlled by controlling the capacitive intercouplings (and, hence, capacitive values of the capacitive elements formed thereof) and the spacing between adjacent ones of the resonators (and, hence, inductive values of the inductive elements formed thereof).
Historically, the component value of the elements comprising such a filter, and, hence, the filter characteristics of the filter formed therefrom, have been controlled in two ways. First, the capacitive values of the capacitive elements formed upon the top surface of the dielectric block have been altered, and, second, the spacings between the adjacent ones of the resonators have been altered.
Alteration of the capacitive values of the capacitive elements formed upon the top surface of the dielectric block is becoming a less viable means of altering the filter characteristics of a dielectric filter as the physical dimensions of such filters are reduced. The capacitive values of such capacitive elements are dependent upon the physical dimensions of the coated areas forming such elements as well as spacings between the coated areas which form the capacitive elements.
As the physical dimensions of the filters are reduced, the physical dimensions of the coated areas which form the capacitive elements must be correspondingly reduced. For such capacitive elements to maintain the same capacitance (as capacitance is directly proportional to surface area, and inversely proportional to distance), the spacings between the coated areas must be reduced.
However, for manufacturing reasons, a minimum spacing is required between the coated areas. Accordingly, alteration of the filter characteristics of such a filter constructed in such manner has become increasingly limited.
Duplexer filters are one such type of dielectric filter commonly utilized to form portions of the circuitry of a radio transceiver. Typically, a duplexer filter is connected between an antenna of the radio transceiver and both the transmitter circuitry and receiver circuitry portions thereof. The duplexer filter comprises a receive portion of a first passband centered about a first center frequency, and a transmit filter portion having a second passband centered about a second center frequency. The first passband of the receive filter portion, and the second passband of the transmit filter portions of the duplexer filter are of passbands of non-overlapping frequencies. Both the receive filter portion and the transmit filter portion are connected to a common antenna; the receive filter portion is coupled to the receiver circuitry of the radio transceiver, while the transmit filter portion is connected to the transmitter circuitry portion of the radio transceiver.
Reductions in the physical dimensions of duplexer filters responsive to increased miniaturization of radio transceivers is limited by the constraints noted hereinabove.
Accordingly, what is needed is a multi-passband filter construction, and means for making such, to be of reduced physical dimensions.